1. Field of the Invention
This invention relates to the recording art and more particularly, to magnetic recording media including various tapes such as video, audio and memory tapes, and other magnetic articles such as magnetic discs, cards and the like.
2. Description of the Prior Art
In magnetic recording media having thin magnetic recording layers of magnetic materials such as cobalt, the thin magnetic layer is usually formed by either a wet method such as electroplating, electroless plating or the like, or a dry method such as vacuum evaporation, sputtering, ion plating or the like. These two types of the methods have the respective advantages as follows. The wet method is advantageous in that the thin magnetic layer is high in reproducibility of magnetic characteristics and the thin magnetic media have, in some cases, a coercive force (Hc) as high as about 1,500 Oe with a good distribution of the coercive force (Hc). That is, the thin layer obtained by the wet method has such a feature that in the magnetization curve, when the magnetic field at a point where a tangent at or near Hc in the curve meets with a residual magnetism Mr is taken as H*, the coercive squareness H*/Hc=S* is relatively high. This high coercive squareness S* leads to the high recording sensitivity (i.e. playback output potential/recording current) with the high playback potential. Accordingly, the magnetic media having such a thin magnetic layer obtained by the wet method exhibit good sensitivity when applied as recording and reproducing media. Moreover, the wet method is advantageous, for example, over a vacuum evaporation technique in that formation of the thin magnetic layer entails little loss of magnetic materials and that a thin magnetic medium having a thin magnetic layer of a given thickness can simply be fabricated. However, it has the disadvantage that the thin magnetic layer formed by the wet method is isotropic in nature in the plane of the layer, so that a residual magnetization to saturation magnetization ratio (Rs) becomes small. In contrast, the thin magnetic film or layer formed by the dry method and particularly by an oblique-incidence vacuum evaporation technique is imparted with the uniaxial magnetic anisotropy with a relatively high Rs ratio and also with the coercive force Hc as high as about 3,000 Oe. However, the evaporation method has the drawback that a quantity of an evaporated material such as Co on a support or base is small relative to a quantity of an evaporated source material with a substantial loss of the evaporated material, leading to high production costs. Moreover, the distribution of the coercive force Hc is so large that the S* value becomes small, resulting in the poor sensitivity when the thin film-type magnetic media made by the evaporation method are used as a recording and reproducing medium. To increase a thickness of the vacuum-evaporated thin magnetic layer for ensuring a satisfactory output potential results in a lowering of magnetic characteristics and particularly in a poor Rs value.
An attempt has been made to overcome the drawbacks of the oblique-incidence vacuum evaporation technique to an extent or to improve the productivity, in which attempt, for example, a base film wound about a roller is subjected first to the vacuum evaporation at a high incident angle and then to the vacuum evaporation at a low incident angle while the roller is rotated in the latter evaporation step. However, by this method is not improved the productivity so far, and the vacuum evaporation at small angles results in a decreasing constant of uniaxial magnetic anisotropy (Ku), coupled with further disadvantages that an angle is established between an axis of easy magnetization and the plane of the layer and that the value S* becomes small. Moreover, if the vacuum evaporation is effected in an atmosphere containing a very small amount of oxygen gas so as to reduce the lowering of Hc owing to the increasing amount of the component formed by the low angle vacuum evaporation, this in turn leads to the disadvantage of reducing the saturation magnetization.